linux-kernel - Re: [PATCH v2 1/3] rust: add UnsafePinned type

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Message-ID: <4ba59b40-7842-4ff3-8ee2-cfa5aa8002bb@gmail.com>
Date: Wed, 30 Apr 2025 18:45:39 +0200
From: Christian Schrefl <chrisi.schrefl@...il.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Sky <sky@...9.dev>, Miguel Ojeda <ojeda@...nel.org>,
 Alex Gaynor <alex.gaynor@...il.com>, Boqun Feng <boqun.feng@...il.com>,
 Gary Guo <gary@...yguo.net>, Björn Roy Baron
 <bjorn3_gh@...tonmail.com>, Benno Lossin <benno.lossin@...ton.me>,
 Andreas Hindborg <a.hindborg@...nel.org>, Trevor Gross <tmgross@...ch.edu>,
 Danilo Krummrich <dakr@...nel.org>,
 Gerald Wisböck <gerald.wisboeck@...ther.ink>,
 linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org
Subject: Re: [PATCH v2 1/3] rust: add UnsafePinned type

On 30.04.25 11:19 AM, Alice Ryhl wrote:
> On Wed, Apr 30, 2025 at 10:36:11AM +0200, Christian Schrefl wrote:
>> `UnsafePinned<T>` is useful for cases where a value might be shared with
>> C code but not directly used by it. In particular this is added for
>> storing additional data in the `MiscDeviceRegistration` which will be
>> shared between `fops->open` and the containing struct.
>>
>> Similar to `Opaque` but guarantees that the value is always initialized
>> and that the inner value is dropped when `UnsafePinned` is dropped.
>>
>> This was originally proposed for the IRQ abstractions [0] and is also
>> useful for other where the inner data may be aliased, but is always
>> valid and automatic `Drop` is desired.
>>
>> Since then the `UnsafePinned` type was added to upstream Rust [1] by Sky
>> as a unstable feature, therefore this patch implements the subset of the
>> upstream API for the `UnsafePinned` type required for additional data in
>> `MiscDeviceRegistration` and in the implementation of the `Opaque` type.
>>
>> Some differences to the upstream type definition are required in the
>> kernel implementation, because upstream type uses some compiler changes
>> to opt out of certain optimizations, this is documented in the
>> documentation and a comment on the `UnsafePinned` type.
>>
>> The documentation on is based on the upstream rust documentation with
>> minor modifications for the kernel implementation.
>>
>> Link: https://lore.kernel.org/rust-for-linux/CAH5fLgiOASgjoYKFz6kWwzLaH07DqP2ph+3YyCDh2+gYqGpABA@mail.gmail.com [0]
>> Link: https://github.com/rust-lang/rust/pull/137043 [1]
>> Suggested-by: Alice Ryhl <aliceryhl@...gle.com>
>> Reviewed-by: Gerald Wisböck <gerald.wisboeck@...ther.ink>
>> Co-developed-by: Sky <sky@...9.dev>
>> Signed-off-by: Sky <sky@...9.dev>
>> Signed-off-by: Christian Schrefl <chrisi.schrefl@...il.com>
> 
>> +config RUSTC_HAS_UNSAFE_PINNED
>> +	def_bool RUSTC_VERSION >= 108800
> 
>> +#![cfg_attr(CONFIG_RUSTC_HAS_UNSAFE_PINNED, feature(unsafe_pinned))]
> 
> Actually, I missed this on the first look. Where is this feature used?
> You only have a re-implementation right now.
Was just a leftover from the previous version, removing the feature and
config flag (to be reintroduced once we want to use the upstream version).

Cheers
Christian